This invention relates to electronic sound generating devices. More specifically, the invention relates to circuits for controlling and driving such devices. Still more specifically, the invention relates to circuits for selecting the particular sounds to be generated by such sound generating devices.
Alarms and audible indicators have achieved widespread popularity in many applications. Of the countless examples available, just a few are sirens on emergency vehicles, in-home fire and carbon monoxide alarms, danger warnings on construction machines when the transmission is placed in reverse, factory floor danger warnings, automobile seat belt reminders, and many more. It is nearly a truism that industry prefers inexpensive but high quality devices to create such alarms and indicator sounds.
Piezoelectric transducers are sound producing electronic devices that are preferred by industry because they are by and large extremely inexpensive, reliable, durable, and versatile. This transducer has the unique property that it undergoes a reversible mechanical deformation on the application of an electrical potential across it. Conversely, it also generates an electrical potential upon mechanical deformation. These characteristics make it highly desirable for sound producing applications. When an oscillating potential is placed across the transducer, it vibrates at roughly the same frequency as the oscillations. These vibrations are transmitted to the ambient medium, such as air, to become sound waves. Piezoelectric transducers can also be coupled to a simple circuit in what is known as a feedback mode, well known in the art, in which there is an additional feedback terminal located on the element. In this mode, the crystal will oscillate at a natural, resonant frequency without the need for continuous applied driving oscillations. As long as the oscillations are in the range of audible sound, i.e., 20 to 20,000 Hertz, such oscillations can produce an alarm or an indicator.
Any periodic oscillation can be characterized by at least one amplitude and frequency. Ordinarily, the amplitude of oscillations of interest in a piezoelectric transducer application will be dictated by the voltage swing applied across the element. By the principles explained above, it is evident that there will be a greater mechanical deformation in the crystal with greater applied voltage. The effect is roughly linear within limits, those limits based in general on crystal composition and geometry. Thus, in the linear region, doubling the voltage swing doubles the mechanical deformation. Doubling the mechanical deformation increases the amplitude of vibrations transmitted into the ambient medium. Increased amplitude of vibrations in the medium causes an increased sound level, the relationship determinable by well known physical equations.
More specifically, when a piezoelectric element possesses two terminals and a driving oscillation is placed across one while the other is clamped to a common potential such as ground, the voltage swing will be at most the amplitude of the oscillations. Thus, if an oscillation of amplitude 5 volts is placed across one terminal, while the other is maintained at 0 volts, the maximum voltage swing will be 5 volts. This effectively caps the achievable decibel level of any sound to a value corresponding to the supply voltage. One could double the supply voltage to achieve double the voltage swing, but this has the disadvantage of added cost, and further is impractical when a piezoelectric audio circuit is to be placed in a unit having a standardized voltage supply such as an automobile. Alternatively, one could use a second supply disposed to provide the same oscillations but in a reversed polarity to double the effective voltage swing. But this approach possesses at least the same disadvantages.
It will be appreciated that when a piezoelectric element possesses two terminals and a driving oscillation is placed across one, and the identical driving oscillation is placed across the other but shifted 180 degrees in phase, the voltage swing will be at most two times the amplitude of the oscillations. Thus, if an oscillation of amplitude 5 volts is placed across one terminal while the other experiences the same oscillation but separated by 180 degrees of phase (half the period of the cycle), then the maximum voltage swing will be 10 volts. Higher sound pressures and louder tones are achievable with a voltage swing of 10 volts than with a voltage swing of 5 volts.
Particularly in alarm applications, what is needed is a loud sound that does not depend on the added circuit complexity of a doubled supply voltage or an additional reversed polarity supply. Loud sounds require relatively high voltages to produce relatively large amplitude vibrations in the transducer. In a special analog circuit, this might not be an obstacle. However, in a circuit containing elements that are safely and reliably operable only in a limited range of potentials, accommodations must be made to insure that those elements do not receive an electrical potential that is too high. Thus, in particular when a loud alarm sound is needed, care must be taken to separate the potentials driving the transducer from the potentials driving the more sensitive circuit elements. For example, integrated circuits often have specifications limiting the recommended power supply to 5 volts DC. If one desires to power a transducer using a supply voltage of 16 volts DC, care must be taken to regulate the power supplied to the integrated circuit.
In both alarm and indicator applications, what is needed is the ability to select different sounds to correspond to different situations. One might wish to distinguish, using discrete tones of differing frequencies, a carbon monoxide alarm from a smoke alarm while still allowing both to use-the same general circuit. In an additional example, one might wish to select one set of tones in an automobile indicator system to represent unfastened seat belts, and yet another set of tones to represent a door ajar, while still allowing both to use the same general circuit. Moreover, it is desirable for such a system to utilize a circuit that inexpensively enables loud sounds to be generated without the need for a doubled or duplicated supply voltage.
It is an object of the invention to provide a circuit for an audio transducer that enables different sounds to be generated that correspond to different operative situations.
Another object of the invention is inexpensively to enable loud sounds to be generated by an audio circuit that overcomes the foregoing disadvantages.
Still another object of the invention is to enable the use of voltage-sensitive components in the same circuit that contains an audio transducer that is disposed to receive large voltage swings.